I'm not gonna say that Sanders didn't sell-out, (cause he kind of did), but saying this will and should taint his legacy forever is counterproductive. Politics is all about compromise. Choosing the lesser of two evils is the reality of this system. If he continued to actively oppose Clinton then he would've only divided the Democrat base from within and ultimately weakened their ability to fight the political battles of the future. This would have only benefited the people and ideologies he opposed. If you lose one election, then you have to regroup and think of how you fight the next battle. Sanders understands this, because he's a career politician who has decades of experience as a Senator and congressman.

Honestly I think this is actually a pretty good indicator that Sanders was never an uncompromising zealot like some people thought he was. At the very least he understands when to take his lumps and move on so he can think about the future. It's a bitter pill to swallow and most politicians don't understand that it has to be done sometimes.